Arthroplasty procedures, such as a total hip replacement, can require the removal of the femoral head and neck, followed by implantation of an artificial hip stem into a reamed portion of the femoral medullary canal. Some hip arthroplasty procedures call for the use of bone cement to secure the hip stem within the medullary canal. For procedures that call for cement, it is generally undesirable to allow the cement to infiltrate the medullary canal to an uncontrolled depth and volume. Therefore, a hip arthroplasty procedure can include the step of placing an obstruction within the medullary canal in an attempt to restrict or block the flow of cement.
Not infrequently, the obstruction is merely a partially hardened or cured ball of cement placed into the canal and held in place by friction fit with the wall of the canal. This makeshift obstruction is easily dislodged by the distal end of the hip stem if the cement ball is not inserted deep enough into the canal. Additionally, the ball of cement is readily displaced when pressurized cement is added to the medullary canal to bind the stem in place. If the cement ball is fractured and/or if it falls beyond a narrow central region of the femur known as the isthmus, the pressurized cement does not properly infiltrate the bone and air pockets or pores are created in the cement. The imperfection laden hardened cement thus provides a poor interlock with the bone and stem and it is susceptible to cracking. Poor mechanical interlock and cement failure causes the stem to loosen. This undesirable occurrence often requires that the joint be replaced in a procedure known as a revision.
Revision surgery and/or procedures requiring a "long" hip stem are especially problematic with an application that calls for pressurized cement. Specifically, the distal end of a revision stem ultimately extends further into the medullary canal than an original "normal" stem because additional bone is cut-away during removal of the original stem in preparation to prepare for implantation of the revision stem, or poor quality bone stock forces a larger stem to be used to secure the stem more distally in the canal to reach better quality bone to achieve implant stability. Whereas the distal end of the original stem may extend to a point before or above the isthmus, and thus above the ball of cement, the distal end of the revision stem may extend beyond the isthmus.
Structures other than cement balls are also known for creating a blockage within a medullary canal. For example, FIG. 1 illustrates a known device 10 including a tapered body 12 having a first end 14, a second end 16, and fins 18 that extend radially from the body. Each fin 18 is resilient and can be flexed toward the first end 14 or the second end 16 of the body 12 as shown in the illustration by dashed lines. Although it is possible to maintain one or more fins 18 in a flexed condition by applying pressure to the fin(s) or placing them in a confined space to elastically deform them, once the pressure is relieved or the device is removed from confinement, the fin(s) will always return to their original position unless they have been plastically deformed. Thus, the fins 18 and the device 10 can be described as only having a single stable state.
In use, a single stable state device 10 can be well suited to the tasks of creating a blockage within a reamed medullary canal 20 above an isthmus region 22 as shown in FIG. 2. It will be noted that the fins 18 are deformed different amounts depending on where they are within the tapered medullary canal 20. The body 12 and the fins 18 can have a thickness such that even when the fins are fully compressed against the body, the device 10 is broader than the isthmus 22 to prevent the device from being readily pushed beyond the isthmus. Thus, in a typical pressurized cement application, the pressurization of the cement does not dislodge the device.
By contrast with an above-the-isthmus application, the device 10 is totally unsuited for beyond-the-isthmus applications as shown in FIG. 3. Specifically, once some of the fins 18 of the device 10 move beyond the isthmus, there is less and less mechanical interlock with the bone and even the application of low pressure causes the plug to be dislodged. Were the device 10 to be deliberately passed beyond the isthmus and then pulled back up into the narrow passage as shown in FIG. 4, the flexed fins 18 would urge the device down and away from the isthmus.